Lithographic printing is a well known art. In lithography a printing master or plate is employed having a printing surface on which the printing image areas are ink receptive whereas the non-printing background areas are water receptive. In the process of printing, an aqueous based so-called fountain solution is applied to the printing surface of the plate and such fountain solution adheres to the water receptive background areas only. An oil base ink is then applied to the printing surface of the plate. Such ink is repelled from the fountain solution containing background areas and adheres to the oil receptive printing image areas only. The printing plate is then brought into contact with paper onto which the image is printed by ink transfer from the printing image areas, as known in so-called direct lithography, or alternatively, as known in so-called offset lithography the printing plate is brought into contact with a rubber blanket onto which the image is offset by ink transfer from the printing image areas and in turn such rubber blanket is brought into contact with paper onto which the image is finally printed by ink transfer from the rubber blanket. The above described steps of applying the fountain solution and ink to the plate are repeated preparatory to each paper copy printing.
Lithographic printing plates can be prepared by numerous methods well known in the art, and one such method is electrostatic or electrophotographic imaging.
Typically in one electrostatic imaging process a lithographic printing plate is prepared by employing a conventional electrophotographic printing element such as for instance a selenium plate for the formation thereon of a latent electrostatic image pattern by methods well known in the art and developing such image pattern by attraction thereto of electroscopic powder which is ink receptive, followed by transferring the powder image onto a water receptive lithographic substrate or plate such as treated paper or grained aluminum and the like on which the transferred powder image subsequently is fused by solvent vapour or heat to form ink receptive printing areas on the water receptive surface of the plate. In another electrostatic imaging process a lithographic printing plate is prepared by developing with toner material comprising ink receptive electroscopic marking particles the latent electrostatic image formed on a so-called electrophotographic binder plate which as is well known in the art typically comprises a paper sheet having on one side thereof a coating consisting of a photoconductor powder such as zinc oxide contained within an insulating resinous binder material, such coating being in itself generally water repellent. The water repellent surface of the plate is then rendered water receptive or converted to become water receptive in the non-image or non-printing areas which are free of ink receptive toner deposit by the application of a so-called conversion solution which again is well known in the art.
In a typical non-electrostatic method of lithographic plate preparation a water receptive grained aluminum or zinc plate is coated with a light sensitive layer and exposed to a light pattern following which the light sensitive layer is developed or processed so that only the printing image areas of such layer remain adhering to the water receptive metal surface to form the ink receptive areas thereon. In another non-electrostatic method of lithographic plate preparation an ink receptive metal surface such as copper is coated with a water receptive metal film such as chromium and such metal surface is subsequently coated with a light sensitive layer which is then exposed to a light pattern and subsequently developed to remove selectively portions of such light sensitive layer and retain same as an etch resist layer in other portions. The surface is subsequently etched whereby in portions free of said etch resist layer the water receptive metal surface is removed in the printing image areas to expose there the underlying ink receptive metal surface, the etch resist layer being subsequently removed to expose the water receptive metal surface.
Whilst electrostatic and non-electrostatic methods of lithographic printing plate preparation have been found useful, there are certain disadvantages in the prior art processes. For instance in the electrostatic process cited wherein the lithographic plate is prepared by transfer of ink receptive powder deposit onto a water receptive substrate the disadvantage is that image resolution is limited by the relatively large particle size of the developing powder and image detail is lost during the powder image transfer step. In the other electrostatic process cited wherein a binder type electrophotographic plate is employed the disadvantage lies in the relatively short run length such as a few hundred to a few thousand copies obtainable from printing plates of this kind, as the substrate employed for the binder coating usually comprises paper. In non-electrostatic methods of lithographic printing plate preparation generally the disadvantages lie in the need for relatively lengthy preparation procedures for such plates and in the need for long exposure times combined with very strong light sources to cause in the light sensitive layer the required effect or contrast for selective processing, and in addition such metal plates are relatively expensive.
It is therefore the general object of this invention to provide a novel lithographic printing plate to overcome the above noted disadvantages.
Another object of this invention is to provide a novel method of preparing a lithographic printing plate.
A further object of this invention is to provide a simple and time saving process of preparing electrostatically a lithographic printing plate.
A still further object of this invention is to provide a novel electrostatically prepared lithographic printing plate which can be exposed at camera speeds.
Yet, still a further object of this invention is to provide a novel electrostatically prepared lithographic printing plate capable of reproducing high resolution images.
Yet another object of this invention is to provide a novel electrostatically prepared lithographic printing plate capable of long run copy production.
Still another object of this invention is to provide an inexpensive lithographic printing plate.
The foregoing objects and other advantages are accomplished in accordance with this invention in the manner as described in the following.